Fix message sample structures

We define pseudo-ASN.1 structures for message-header.md, message-body.md, and message-footer.md, however these are not proper ASN.1 structures. This could cause confusion for anyone reading the spec.

Acceptance criteria:
- get team alignment on how we want to describe example structures in the spec
- update these documents so that the example structures do this (i.e. if team wants to represent valid ASN.1 structures, fix these examples so that they are valid ASN.1)

Start by reading the example structures in message-header.md, message-body.md, and message-footer.md, then review the issue's acceptance criteria and seek team alignment on the intended notation. Done means all three documents consistently use the agreed representation and, if valid ASN.1 is chosen, their examples are valid ASN.1 structures.

Written by the indexing model from the issue text.